The present invention concerns an electrode for the measurement of liquids. The electrode which has a metal sensing element contained in a holder, is intended primarily for the determination of pH, but can also be used for the measuring of P.sub.O.sbsb.2 and P.sub.CO.sbsb.2.
The measurement of pH and other quantities in liquids is very important in many connections. Such measurements are carried out in a routine fashion in many laboratories and in technical operations. Measuring electrodes of this type have also been developed for medicinal applications.
The best known and most widely used pH electrodes have been glass electrodes. The latter must be made with very thin glass walls and are therefore very sensitive to mechanical stress. This makes necessary special handling and safekeeping, which limit their exploitation. The glass electrode, moreover, cannot be produced with the small, precise dimensions necessary for many uses if the required measuring precision is to be maintained.
Metal electrodes have greater mechanical stability. Such electrodes with sensors of iridium, palladium, antimony, and platinum have been developed for various purposes, e.g., for the measuring of pH in soil. Another example would be miniaturized electrodes for the in vivo measuring of pH in blood. These electrodes, however, have proved to have poor stability and are difficult to reproduce, and the variations between different electrodes are considerable. They are, moreover, at least in certain cases, sensitive to impurities from other metal ions in the testing liquid, and they can also show sensitivity to touch of the electrode surface and to agitation of the testing liquid. The known technique is described, for example, in U.S. Pat. No. 2,416,949, German Patent Application 2,333,641, and Swedish Pat. Nos. 384,921 and 384,922.
The purpose of the present invention, therefore, is to provide a metal electrode which is suited for miniaturization. It is also to provide an electrode which possesses stability and is reproducible over long and short periods. The electrode is also intended to withstand normal handling and to be minimally sensitive to variations in normally occurring impurities in the testing liquid.
It has proved possible to attain these qualities and other advantages, as, for example, reproductiveness between individual electrodes, by means of metal electrodes according to the present invention which are characterized in that the metal sensor is a monocrystalline metal with only one plane crystal face exposed to the liquid being tested for pH, P.sub.O.sbsb.2 or P.sub.CO.sbsb.2. Electrodes according to the present invention make possible exact long-period and short-period measurements of pH in smaller testing amounts than has been the case hitherto with known polycrystalline metal electrodes. Electrodes according to the invention, moreover, require considerably less calibration than is necessary with glass electrodes. A particular embodiment of the present invention is constituted by the use of "whiskers", which are single crystals with only one dislocation.